When an oil well is drilled it passes directly through an oil reservoir from which oil will be produced to the surface. A bore is drilled into the oil reservoir and a production string is introduced into the bore. Production tubing string is made up of individual tubing sections approximately 9.1 meters (30 feet) long. Attached to the top end of each tubing section is a coupling with two female thread forms to allow corresponding male threads on the end of the tubing sections to be threaded together to create one continuous tubing string.
The rock which makes up the oil reservoir may vary in type and physical characteristic, but the main characteristic of interest is the permeability of the rock. The permeability determines the ease with which the oil can flow through the rock and into the oil well.
Certain rocks such as sandstone have a relatively even permeability and are called homogeneous. Oil can flow through the homogeneous rock at a relatively even pace and will be produced evenly across the drilled section of reservoir. Other reservoir rocks such as limestone and chalk can be heavily, naturally fractured and vary greatly in permeability. These rocks are known as heterogeneous. Oil from a heterogeneous reservoir will produce mainly from the areas of highest permeability where the fractures occur.
Even through the oil well may be drilled through a considerable length of the oil reservoir, the high permeability zones may account for only 10-15% of the length of the drilled reservoir section. If allowed to produce directly into the drilled hole and production tubing string, the oil will never be produced from the remaining 85-90% of the drilled section thus reducing the efficiency of the oil well.
Another problem is that directly beneath the oil reservoir there is typically a layer of naturally occurring water. When a well is drilled the aim is to produce as much oil as possible and to limit the amount of natural water produced. Over time as the oil is depleted, it is replaced by the natural water seeping up from the rock below. In a homogeneous reservoir the water may rise slowly and evenly, prolonging the time before water eventually breaks through into the well bore. In a heterogeneous reservoir the mixed permeability of the reservoir and the natural faulting may allow water to be produced almost immediately at the expense of oil production.
To overcome these problems of producing oil from a heterogeneous oil reservoir a number of mechanical components have been designed to control the flow of oil into the production tubing string. Historically the oil was allowed to flow from the hole drilled through the reservoir directly into the production tubing string via the open end of the tubing string or via holes drilled evenly along the length of the tubing string. This method of production made no difference to the permeability of the reservoir and resulted in production from a limited portion of the drilled section leading to early water break-through.
It was discovered that if the flow of oil from the reservoir could be mechanically restricted as it passed into the tubing string, the resulting back pressure created would allow sections of the reservoir with lower permeabilities that would not normally get a chance to produce, due to the higher permeability zones, to contribute to the well's production. This effectively increased the oil producing area of the reservoir and extended the time before eventual water break-through.
Devices which invoke this effect come in a variety of forms and have the common feature of restricting flow by creating a pressure drop as the oil passes through them. The restriction can take the form of a series of orifices or a tortuous flow path. The devices are provided in the production tubing string and are spaced out at intervals across the reservoir section. As the oil produces it will pass out of the oil reservoir rock and fill the annular space between the bore hole drilled through the reservoir and the outside of the production tubing string. Thereafter, the oil will flow towards the flow restriction devices and enter the production tubing string.
The substantially continuous annular space between the bore hole drilled through the reservoir and the outside of the production tubing string can be effectively partitioned into a number of compartments by the location of collars around the production tubing at regular intervals. These collars are spaced from the flow restriction devices in a longitudinal direction and each compartment may comprise at least one flow restriction device.
These collars are commonly known as mechanical open-hole packers, and these packers form a barrier between adjacent compartments. In the event that water was to break-through to one compartment, the packers serve to isolate adjacent compartments and prevent the water flowing into adjacent compartments in the annular space between the bore hole drilled through the reservoir and the outside of the production tubing string.
The packers are fitted to, and sealed around, a completion liner and then inserted into the well. Existing mechanical open-hole packer seal technology uses packers formed of a seal of deformable elastomer material. After the completion section is placed in the reservoir, the packers are set against the open-hole section of the reservoir by pressurising a piston seal assembly which serves to drive a mechanism which transversely deforms the packer seal along the direction of the completion liner. Actuating the packer in this manner causes it to deform in a transverse/radial direction forming a seal between the completion liner and the open-hole section of the reservoir.
It is also known for packers to be formed of a material which is arranged to expand, i.e. swell, as a result of contact with, for example, a liquid found in, or in the vicinity of, the reservoir, so that the packer expands to seal the space between the completion liner and the well wall.
European Patent Application No 08104394.5 relates to mechanical packers suitable for the applications described above.
These mechanical packers can be set by pumping fluid into the production tubing from the surface to increase the pressure within the production tubing compared to that exterior to the tubing. The pressure within the production tubing is maintained at a specific level for a period of time to ensure that all mechanical packers are set. However, the flow restriction devices are, in their simplest form, apertures formed in the walls of the production tubing and through which the fluid being pumped into the production tubing can escape to the exterior to the tubing, thereby causing a loss in pressure within the production tubing. As will be appreciated, these leaks can cause difficulties in maintaining the required pressure within the production tubing and can increase the time required to set the mechanical packers.
The mechanical packers can also be set by way of a setting tool. However, the use of such a tool represents extra operational expense because each packer has to be set individually, and if there are, for example, ten mechanical packers in a well, this can typically equate to an extra twenty-four hours of operation.